Experimental Thermodynamic Investigation on the Refrigerant Charge in a Transcritical CO2 Electric Bus Air Conditioning System

Haidan Wang,Paride Gullo,Yulong Song,Xiang Yin,Feng Cao,Shengbo Li

doi:10.3390/app11125614

Haidan Wang, Paride Gullo + Show 4 more

Open Access

https://doi.org/10.3390/app11125614

Copy DOI

Abstract

Due to its considerable impact on climate, bus air conditioning systems are being pushed to take a new and sustainable path. Electric buses relying on transcritical CO2 air conditioning units are perceived to be eco-friendly and future-proof solutions to achieving such a target. However, in order to have highly efficient air conditioning systems, the CO2 charge needs to be optimized. In this paper the energy and exergy-based analyses were performed to investigate the effect of normalized refrigerant charge on the system performance by using a test rig of a transcritical CO2 air conditioning unit for an 8 m electric bus. Results showed that the normalized refrigerant charge range of 0.248~0.336 was recommended in order to ensure the maximum coefficient of performance (COP). In addition, in sufficient charge conditions, the optimal COP, cooling capacity and exergy efficiency were 1.716, 18.97 kW and 29.79%, respectively, under the standard refrigeration condition of 35 °C/27 °C. As the ambient temperature rose from 35 °C to 40 °C, the COP, cooling capacity and exergy efficiency decreased by 16.03%, 10.90% and 12.22%, respectively. Furthermore, the exergy efficiency was found not to be sensitive to slightly insufficient charge, whereas overcharge was observed to be even beneficial to exergy efficiency under the condition of ensuring the maximum COP. In addition, insufficient refrigerant charging seriously affected the irreversible losses in the indoor and outdoor heat exchangers, whereas slight overcharge had little effect on the component exergy efficiency. Finally, the need to improve the CO2 compressor efficiency to enhance the system performance was revealed.

Highlights

Introduction iationsElectricity as an energy vector for vehicle propulsion promotes the use of renewable and carbon-free energy sources in addition to favoring energy supply security as well as lowering air pollution
As the ambient temperature rose from 35 ◦ C to 40 ◦ C, the coefficient of performance (COP) and cooling capacity of the system decreased by 16.03% and 10.90%, respectively
As the normalized refrigerant charge (NRC) was 0.204, the COP and Qcooling decreased by 1.39% and 4.06% at 35 ◦ C, by 3.56% and 5.94% at 38 ◦ C and by 7.04% and

Summary

Introduction

Electricity as an energy vector for vehicle propulsion promotes the use of renewable and carbon-free energy sources in addition to favoring energy supply security as well as lowering air pollution. At present, hydrofluorocarbons (HFCs) are the most widely used refrigerants for bus air conditioning systems, especially. R407C and R410A [3], which have a high value of global warming potential (GWP). The wide adoption of the Kigali Amendment will result in HFC cut by around. 85% before 2050 on a global basis, leading low-to-zero GWP refrigerants to take center stage in transport sector. CO2 is a widely used natural working fluid (R744) with negligible GWP, zero ozone depletion potential (ODP), low cost and excellent thermo-physical properties in addition to being non-flammable and non-toxic [4].

Methods

Results

Conclusion